Conventionally, apparatus of this type include an X-ray CT (Computed Tomography) apparatus which revolves an X-ray tube and an X-ray detector about the body axis of an object under examination. In recent years, a flat panel detector (hereinafter abbreviated as “FPD” where appropriate) has been used as the X-ray detector. The X-ray tube and FPD are revolved about the body axis by a C-shaped arm. A sectional image is reconstructed by carrying out a back projection arithmetic process on projection data detected by the FPD. Also in an X-ray CT and other section radiography (what is called “non-CT type”), a back projection arithmetic process is carried out after a filtering process or the like on projection data (see Patent Document 1, for example).
The FPD has a plurality of X-ray sensitive detecting elements arranged two-dimensionally on a detecting plane thereof. The element size of each detecting element of this FPD is considerably smaller than the element size of a conventional detector dedicated to gantry type CT. Therefore, in order to create an image with a thickness corresponding to conventional CT, it is necessary to add a plurality of pixels together, and regard and handle them as one large pixel. In this regard, it has been conventional practice to add images reconstructed finely, or to carry out reconstruction after applying an addition average filter in the direction of the body axis to an original image. In order to add finely reconstructed images as in the former, it is necessary to reconstruct many images, which results in an extended processing time. Thus, an addition average processing such as the addition average filter as in the latter, and reconstruction is carried out using a value derived from the addition average processing, thereby to shorten processing time.